I. Field of the Invention
This application in general relates to apparatus which makes use of certain compositions, generally referred to as phase change materials (PCM's), which change their phase, generally between solid and liquid phases, and thereby store heat energy during such change. More particularly, it refers to apparatus for storing the cooling capacity--actually heat below room temperature--which apparatus makes use of a cooling tower to provide a supply of chilled fluid, usually water, to PCM's.
II. The Prior Art
It has recently become recognized that, while there may be a need for storing heat generated during daytime hours by solar energy, there is an even more immediate need for storing what might be perceived as coolness, i.e., heat energy at temperatures substantially lower than body temperature or room temperature. A class of compositions, which are known as phase change materials, act to store cooling capacity when they have a freezing-melting point below room temperature. As such PCM's melt, they take up heat from the surrounding ambience, and in so doing cool substances within which they are in close contact. Thus, where there is a tank containing packaged PCM's that have a low freezing-melting point, which is sometimes a range of 2 or 3.degree. F., circulating water through the tank and into close proximity, but not contact with the PCM's results in that water being chilled approximately to the melting point of the PCM's.
It is thus recognized that cooling capacity can be stored utilizing PCM's in the form of salt hydrates and additives and modifiers thereto to form eutectoid compositions the freezing-melting points of which can be controlled to a predetermined value. As examples of such PCM's, homogeneous mixtures based on the use of sodium sulfate decahydrate and other ingredients have been described by Dr. Maria Telkes in U.S. Pat. Nos. 2,677,664 and 3,986,969. While such eutectoid salt compositions have not achieved an outstanding degree of economic success at this time, it is believed that their use will become much more widespread in the future.
In arranging PCM's in a tank through which water or other fluid is to be supplied first to freeze and then to be chilled upon melting of the frozen PCM's, it will be apparent that phase change materials such as those including Glauber's salt cannot be permitted to come into direct contact with the fluid, e.g., water. These salt compositons, which rely on a solid-liquid change of phase, are separately packaged. In general, it has been found most advantageous to house such PCM's in separate containers to form packages that will be self-stacking within a tank and which will nest one into the other. In particular, one such package that has been found to be particularly advantageous for use in containing phase change materials is that disclosed in my co-pending application, Ser. No. 696,529, filed Jan. 30, 1985 and entitled, Nestable, Stackable Containers. That application describes nestable, stackable containers adapted to hold PCM's based on sodium sulfate decahydrate in a tank in such a manner that water or other fluid entering the tank will come into good heat exchange relationship with the salt compositions so packaged but, of course, not into direct contact with the salts. By such stackable, nestable containers arranged within the tank, the fluid that is brought into heat exchange relationship with the packaged salts in the tank either gives up its coolness to freeze the salts or, if the fluid is at a temperature greater than the melting point of the salts, to acquire coolness from the frozen salts and thereby be chilled approximately to the melting point of the salts.
Another part of the apparatus which together comprises the present invention is a cooling tower, likewise well-known in the art. In general, a cooling tower is a tower-like device in which atmospheric air circulates in direct or indirect contact with warmer water and thereby cools the water. Cooling towers are often used as heat sinks in refrigeration or steam power generation. They may also be used in any process in which water is a vehicle for heat removal and when it is desirable to make final heat rejection to the atmosphere. Basically, water, acting as the heat-transfer fluid, gives up heat to atmospheric air and, thus cooled, is recirculated through a system, affording economical operation of particular process.
Two basic types of cooling towers are commonly used. One transfers the heat from warmer water to cooler air primarily by evaporation of a portion of the water; it is known as an evaporative or wet cooling tower. The other type of tower transfers heat from the warmer water to cooler air by a sensible heat-transfer process and is known as a non-evaporative or dry cooling tower. While dry and wet cooling towers have their own advantages and disadvantages, the evaporative cooling tower has the potential for cooling the water therewithin to an approach temperature less than 5.degree. F. above the wet-bulb temperature of the ambient air. A non-evaporative cooling tower does not usually have the capacity to cool the warm circulating water to within 5.degree. to 10.degree. F. of the temperature of the incoming air. Consequently, since in the present invention it is desirable as a practical matter to have the cooling tower cool the water therewithin to as low a temperature as possible, evaporative cooling towers appear to be the most easily adapted to the present apparatus, despite the fact that there is some loss of water vapor and drift from the tower. Other than the fact that evaporative cooling towers are preferred, the particular structure of the tower, itself, e.g., whether it uses atmospheric cooling, natural draft or mechanical draft to produce air-water contact, is not deemed of great importance at this time. What is important is that the cooling tower employed be such as will most efficiently lower the temperature of the water flowing therein to a temperature as close as possible of that of the ambient air.
Finally, the prior art also recognizes the use of water to cool the condenser of a chiller itself utilized to cool a space, such as a building. This can be accomplished by cooling the condenser water in a cooling tower and then exchanging the cooling capacity of that water in a heat exchanger through which the water from the chiller loop of a building also passes. Alternatively, the condenser water can also be used directly in the building water loop with a filtration system provided to strain foreign matter from the condenser water. Such a water system is disclosed in U.S. Pat. No. 3,995,443, and is marketed by Thermocycle International, Inc., of New York, N.Y. under the trademark Strainercycle.
Both of these approaches to the use of condenser water chilled in a cooling tower to thereafter provide chilled water to air condition a building, suffer from serious defects. It will be principally apparent that the requirement for cooling a building often will not coincide with the availability of water cooled by environmental (water tower) cooling. Thus, tower water will often be available only when the chiller is not in active operation, generally during the middle of the night when there is less need to provide cooling to a building, which may carry a heat load only during business hours.
It is, therefore, an important feature of the present invention is to store the cooling capacity contained in environmentally chilled water for use at another time when there is an air conditioning requirement that would normally have to be satisfied through mechanical means, the production of which has a high energy requirement. By using thermal energy storage, one can extend dramatically the number of hours environmentally chilled water can be substituted for mechanically refrigated water.
With regard to the mode in which the cooling capacity of environmentally chilled water can be stored, the use of eutectoid salt compositions has marked advantages over ice storage systems and chilled water storage systems. If not wholly inappropriate, ice storage systems are severly limited by the low temperatures they require to function. Cooling towers that chill condenser water simply do not produce water temperatures below 32.degree. F. Chilled water storage is likewise inappropriate due to its space requirements and blending problems normally associated with it. Moreover, the inappropriateness of ice storage and chilled water storage for taking advantage of tower water coolness is emphasised by the fact that environmentally cooled water will vary to some extent as ambient dry and wet bulb temperatures vary. In a chilled water storage system, for example, varying inlet water temperature would exacerbate the blending problems markedly associated with chilled water storage. Hence, I am not aware of the employment of any kind of storage means for retaining the coolness of environmentally cooled water, although the concept of storing such thermal energy is most attractive as a low-energy means to air condition space in lieu of mechanical refrigeration.